1. Field of the Invention
This invention relates generally to an ionographic printing machine, and more particularly to a scheme for controlling the toner concentration within the developer mixture by sensing the charge of the particles which develop a latent image patch having predefined characteristics.
2. Description of the Prior Art
In general, the process of ionographic printing includes charging of an electrostatic member to a substantially uniform potential to sensitize the surface thereof. The charged surface of the electrostatic member is subsequently exposed to a charge pattern representative of the image to be produced, thereby forming an electrostatic latent image. The latent image is then developed by bringing developer material into contact therewith. Generally, the developer material is composed of toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image forming a toner image on the electrostatic member, which is subsequently transferred and fused to a print sheet.
More specifically, as toner particles are depleted from the developer material, additional toner particles must be added thereto. Many types of toner concentration regulating systems are known in the art. For example, U.S. Pat. No. 4,619,522 to Imai teaches the use of a reference pattern, with a predetermined reflectance, that is developed. Subsequently, the density of the developed pattern is detected and used to regulate the replenishment of toner to the developer.
Furthermore, U.S. Pat. No. 4,434,221 to Oka discloses a method of utilizing a reference latent image to measure the current flow between the developing sleeve and the photoreceptor drum during development of the reference image. Subsequently, the amount of toner needed for replenishment is controlled, based on the current value measured. Oka further characterizes this method as inferior, because, the variation in current value due to toner concentration is exceeded by the variation due to the amount of toner adhering to the reference image.
In addition, U.S. Pat. No. 4,492,179 to Folkins et al., incorporated herein by reference, teaches the sensing of the charge of the toner particles being transferred to the latent image, and controls the addition of toner to the developer as a function of that measurement. Folkins et al. also discloses the limitations of the marking particle dispense control system, relating to toner dispensing assumptions, wherein the rate of dispense must remain constant over the life of the system. More specifically, any variation in the toner mass dispensed for a given electrical input will manifest itself proportionally as a shift in the relationship between the toner dispense rate and the bias current required for the developed toner charge. Unfortunately, these limitations can lead to the implementation of a development system that is prohibitively expensive to be utilized in low volume personalized printing systems, such as an ionographic printing machine.
It is therefore an object of the present invention to provide an apparatus to improve the method of regulating the concentration of toner particles in the developer mixture. It is another object of the present invention to provide an improved apparatus for regulating the dispensing of toner into a developer mixture in response to the current generated by the developer toner and in proportion to the current generated by the periodic development of a latent image patch having a set of predefined and controlled characteristics. It is yet another object of the present invention to provide a means for periodically generating a latent image patch, whereby said patch consistently meets a set of predefined characteristics. It is a final object of the present invention to increase the allowable latitude of the development station components so as to reduce the overall cost of the components without impact to the image development capability or output print quality of the machine.
Further advantages of the present invention will become apparent as the following description proceeds and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.